This invention relates to a rotary position transducer and method of mounting the same.
U.S. patent application Ser. No. 08,867643, filed Jun. 2, 1997 and assigned to the assignee of this invention, describes an electric steering apparatus for a motor vehicle including a rack bar supported on an axle housing of the motor vehicle for back and forth linear translation and linked to steerable wheels on the axle housing, a rotatable pinion shaft on the axle housing having a pinion gear thereon meshing with a rack gear on the rack bar, and an electric motor on the axle housing which steers the steerable wheels by rotating the pinion shaft. Because the pinion shaft is directly connected to the steerable wheels through the pinion gear and the rack bar, the direction of the steerable wheels may be monitored by monitoring the angular position of the pinion shaft. To that end, a rotary position transducer on the axle housing includes a transducer housing rigidly attached to the axle housing, a rotor rotatable as a unit with the pinion shaft, and a stator around the rotor rigidly attached to the transducer housing. The positions on the axle housing of the transducer housing and the pinion shaft may vary laterally relative to each other within allowable tolerance ranges attributable to ordinary manufacturing variables. When the pinion shaft and the transducer housing are at opposite ends of their respective allowable tolerance ranges, the transducer rotor may physically interfere with the transducer stator. To avoid such interference, the allowable tolerance ranges must be minimized. Such minimization, however, contributes to increased manufacturing costs. A rotary position transducer and method according to this invention is an improvement over prior rotary position transducers and methods transducers and methods in that interference between a rotor and a stator thereof is positively foreclosed at a cost which is less than the cost of minimizing the aforesaid manufacturing tolerance ranges.
This invention is a new and improved rotary position transducer and method of mounting the same. The transducer includes a housing having a disc-shaped end plate thereon, a guide in a main aperture in the end plate, a plurality of frangible coupling elements rigidly supporting the guide on the end plate in the main aperture, a stator rigidly supported on the transducer housing behind the end plate, and a rotor rigidly attached to the guide and supported inside of the stator with an annular gap therebetween. The transducer is mounted on a structural member around a shaft rotatably supported on the structural member by first mounting the guide on the shaft, then rigidly clamping the transducer housing to the structural member, and then applying torque to the shaft to fracture the frangible coupling elements and release the rotor for rotation with the shaft relative to the transducer housing and the stator. A radial depth dimension of the annular gap between the rotor and the stator exceeds the maximum lateral departure of the shaft from a rotation axis of the structural member, i.e. the maximum lateral xe2x80x9crunoutxe2x80x9d of the shaft, thereby to positively foreclose physical interference between the rotor and the stator after the coupling elements are fractured. In a preferred embodiment, the end plate and the guide are made of molded structural plastic and the frangible coupling elements are integral radial spokes between the guide and the end plate.